Meanwhile, heat oil in heavy large skillet over high heat. Add remaining 6 cups chopped onions. Sauté until beginning to brown, about 10 minutes. Reduce heat to medium; sauté until onions are deep brown, stirring often, about 45 minutes. DO AHEAD: Stew and onions can be made 1 day ahead. Cool, cover, and chill separately. Rewarm each over low heat before continuing.